Getting a poor night's sleep does more than leave you groggy the next morning. Our sleep patterns have a strong influence on our mental and physical health. In Internal Time: Chronotypes, Social Jet Lag, and Why You're So Tired (Harvard University Press, 2012), Till Roenneberg reveals that chronically sleep-deprived people are more likely to smoke, gain weight and become sick. The reason, he explains, is that our largely indoor and sedentary lives confuse our internal clocks. To combat the malaise, he recommends spending more time in the sun by walking to work or eating lunch outdoors.

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